Recent years have seen rapid progress in size- and weight-reduction of mobile information terminals such as cellular phones, laptop personal computers, and smart phones and batteries used as the driving power sources for these devices have been required to achieve further higher capacities. A nonaqueous electrolyte secondary battery that is charged and discharged through migration of lithium ions between a positive electrode and a negative electrode has a high energy density and a high capacity; thus, nonaqueous electrolyte secondary batteries are widely used as the driving power sources of such mobile information terminals.
Mobile information terminals tend to consume more power as they are equipped with more and more functions such as a moving picture playing function and a gaming function and thus further higher capacities are desirable. The capacity of a nonaqueous electrolyte secondary battery can be increased by increasing the capacity of an active material, increasing the fill density of the active material used per unit volume, and increasing the charge voltage of the battery. However, increasing the charge voltage of the battery accelerates decomposition of the electrolyte, which is a problem. In particular, when a battery is stored at a high temperature or subjected to a repeated charge-discharge cycle at high temperature, a problem of a decrease in discharge capacity arises.
In view of these, there has been a proposal of using, as a positive electrode active material, a mixture of lithium cobaltate and lithium nickel cobalt manganate, as described below.
PTL 1 proposes a nonaqueous electrolyte secondary battery that includes a positive electrode, a negative electrode, and an electrolyte, in which the positive electrode contains, as a positive electrode active material, at least LixCoO2 and LiyNisCotMnuO2 (M represents B, Mg, or the like) and the content of LiyNisCotMnuO2 relative to the total content of LixCoO2 and LiyNisCotMnuO2 is 10 to 45% by weight. PTL 1 shows that a nonaqueous secondary battery that has a high capacity, high safety in the event of overcharge, good high-temperature storage properties, and generates little gas under normal operation conditions can be obtained by using this positive electrode.
PTL 2 proposes a positive electrode active material for a lithium secondary battery in which a surface is coated with AlF3, ZnF2, or the like. PTL 2 shows that a phenomenon of degradation of battery performance, such as cycle properties and in particular performance at high voltage and high rate, can be prevented by using this positive electrode active material.